Problem: Which of these numbers is composite? ${11,\ 37,\ 39,\ 73,\ 89}$
A composite number is a number that has more than two factors (including 1 and itself). 11, 37, 73, and 89 each have only two factors. The factors of 39 are 1, 3, 13, and 39. Thus, 39 is the composite number.